1. Field of the Invention
The present invention relates to a composite ceramic sintered material containing silicon carbide, silicon nitride, free carbon, and the like, and to a process for producing the same. More specifically, the invention relates to a composite sintered material having large strength, toughness and excellent sliding property.
2. Description of the Prior Art
Non-oxide type ceramics as represented by silicon carbide and silicon nitride are now drawing attention as materials having hardness, strength, toughness and chemical stability superior to those of other ceramics and metals, and have already been put into practical use as structural materials and wear-resistant materials, and have further been developed in recent years into structural materials for heat engines such as gas turbines. These ceramics have further been used as mechanical sealing parts, bearing parts, valve parts for chemicals, etc.
The silicon carbide is obtained by being heated in an inert atmosphere at a temperature of 1800.degree. to 2200.degree. C. by adding a boron-containing compound such as B.sub.4 C or carbon as a sintering agent, and study has been forwarded to use it as a structural material for high-temperature applications because of its excellent strength, excellent hardness, excellent abrasion resistance, and very loss of strength at high temperatures.
On the other hand, the silicon nitride is heated in an nitrogen atmosphere at 1600.degree. to 200.degree. C. to increase its density by adding Al.sub.2 O.sub.3 or oxides of elements of the Group 3a of the periodic table as a sintering agent, since the silicon nitride itself is sintered very difficulty. The silicon nitride exhibits excellent toughness and resistance against thermal shocks though it lacks strength at high temperatures as compared with the silicon carbide.
In an attempt to obtain the silicon nitride and silicon carbide in a composite form, therefore, there has heretofore been proposed as disclosed in, for example, Japanese Laid-Open Patent Publication No. 264364/1985 to join a sintered material of silicon carbide and a sintered material of silicon nitride together, to bake a mixture of the silicon carbide and the silicon nitride, or to form a mixture layer of the silicon nitride and the silicon carbide in at least the surface layer by making metal silicon present in the sintered product of silicon carbide by the reaction sintering method followed by nitriding in an nitrogen atmosphere.
When the sintered materials are to be joined together, however, a suitable joining agent is necessary to join them together. Moreover, the junction portion has a strength which is smaller than the strengths of the sintered materials making it difficult to obtain properties inherent in the respective sintered materials. Even in the case of a simple mixing method, it is difficult to homogeneously disperse them together which therefore is poorly sintered, leaving much room for improvement.
Furthermore, the reaction sintering method has a problem in that it is virtually difficult to completely nitride the metal silicon; i.e., the metal silicon that remains in the sintered material deteriorates the strength. Besides, the surface layer of the obtained sintered material consists of a mixture phase of silicon carbide and silicon nitride, making it difficult to obtain properties inherent in the silicon nitride.
As for applications as slider members, either the silicon nitride alone or the silicon carbide alone does not exhibit sufficient sliding property. Therefore Al.sub.2 O.sub.3 or an oxide of an element of the Group 3a of periodic table are added as sintering agent to the powder of silicon nitride or to the powder of silicon carbide, or carbon and B.sub.4 C or the like are added and, at the same time, a solid lubricant such as graphite or BN is added, and they are heated in vacuum or in an inert atmosphere, in order to homogeneously disperse the solid lubricant in the matrix consisting of the silicon nitride or silicon carbide thereby to enhance slider property on the surface of the sintered material.
The slider property enhances with an increase in the amount of the solid lubricant in the surface layer of the sintered material. However, the solid lubricant that is added in large amounts is detrimental to the density of the sintered material itself and deteriorates the strength of the ceramics itself which is a so-called skeltal material, causing the slider member to be cracked or broken. Therefore, limitation is imposed on the amount of adding solid lubricant.
From the standpoint of production, furthermore, the solid lubricant must be homogeneously dispersed. Depending upon the cases, the solid lubricant in the sintered material becomes a cause of destruction of the sintered material and deteriorates the strength. With the sintered material consisting of the silicon nitride as a matrix and in which a solid lubricant is dispersed, furthermore, a metal oxide that is added as a sintering agent and the exists in the grain boundary of the silicon nitride crystals deteriorates the resistance against chemicals, and the range of application is limited.